Only around one in 10,000 people has the ability to correctly classify a note simply by hearing it. This phenomenon is called perfect, or absolute, pitch, and has been made famous by the well-known composers who are believed to have possessed such talents, such as Mozart and Beethoven.

This is a great example of how our immediate surroundings and perceptions can change the way in which we view the worldStephen Hedger, University of Chicago

Graduate student Stephen Hedger, from the University of Chicago, US, had perfect pitch identified by objective tests. He explained what it meant.

"I'm able to name any musical note in isolation without the aid of a reference note. Someone with perfect pitch would be able to tell you a car alarm is honking in F sharp, for example. Generally it enables people to identify notes across a wide variety of octaves."

Mr Hedger was tricked by his colleague who secretly adjusted the pitch on an electronic keyboard as he was playing a tune. The notes were made flat by 33 cents - which is one-third of the distance between adjacent keys on a piano.

When the note was shifted back to its original correct key, it sounded drastically sharp to Hedger, who explained he found it "shocking" that he had not noticed the change.

A similar model was tested on 27 students with perfect pitch. They were played a piece of music for 45 minutes which was gradually changed over time to become flatter.

The subjects then perceived the flattened music as in tune, while the in-tune notes were perceived as slightly sharp.

"In the literature, perfect pitch is talked about as a fixed ability, so it was quite surprising to find that as little as 45 minutes of de-tuned music could temporarily shift note categories," Mr Hedger told BBC News.

People with perfect pitch did not identify out of tune tones after hearing music that was slightly flat

"What this points to is the malleability of the human brain. Relatively brief exposure to flattened music is able to rearrange what was thought to be a very long-term and stable note category.

"This is a great example of how our immediate surroundings and perceptions can change the way in which we view the world."

Prof Howard Nusbaum, another author of the study also from the University of Chicago, said these findings could tell us whether or not individuals could also improve their pitch perception as opposed to "de-tuning it".

"We are finding out more and more about how our brains are equipped to learn new things at any age and not limited by abilities previously thought to be available only from the time of birth," he said.

The researchers will now look at whether or not they can improve people's perception of pitch.